Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products.
Hydrotreating is a hydroprocessing process used to remove heteroatoms such as sulfur and nitrogen from hydrocarbon streams to meet fuel specifications and to saturate olefinic compounds. Hydrotreating can be performed at high or low pressures, but is typically operated at lower pressure than hydrocracking.
Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking unit may contain one or more beds of the same or different catalyst.
Slurry hydrocracking is a slurried catalytic process used to crack residue feeds to gas oils and fuels. Slurry hydrocracking is used for the primary upgrading of heavy hydrocarbon feed stocks obtained from the distillation of crude oil, including hydrocarbon residues or gas oils from atmospheric column or vacuum column distillation. In slurry hydrocracking, these liquid feed stocks are mixed with hydrogen and solid catalyst particles, e.g., as a particulate metallic compound such as a metal sulfide, to provide a slurry phase. Slurry hydrocracked effluent exits the slurry hydrocracking reactor at very high temperatures around 400° C. (752° F.) to 500° C. (932° F.). Representative slurry hydrocracking processes are described, for example, in U.S. Pat. Nos. 5,755,955 and 5,474,977.
During an SHC reaction, it is important to minimize coking. It has been shown by the model of Pfeiffer and Saal, PHYS. CHEM. 44, 139 (1940), that asphaltenes are surrounded by a layer of resins, or polar aromatics which stabilize them in colloidal suspension. In the absence of polar aromatics, or if polar aromatics are diluted by paraffinic molecules or are converted to lighter paraffinic and aromatic materials, these asphaltenes can self-associate, or flocculate to form larger molecules, generate mesophase and form coke.
Solids in SHC product not dissolved by toluene include catalyst and toluene insoluble organic residue (TIOR). TIOR includes coke and mesophase and is heavier and less soluble than asphaltenes which are soluble in toluene but not heptane. Mesophase formation is a critical reaction constraint in slurry hydrocracking reactions. Mesophase is a carbonaceous, liquid-crystal material defined as anisotropic particles present in pitch boiling above 524° C. The presence of mesophase can serve as a warning that operating conditions are too severe in an SHC reactor and that excessive coke formation is likely to occur under prevailing conditions.
This coking can be minimized by the use of an additive or controlled by lowering reaction temperature. However, temperature reduction can also reduce conversion of poorer feeds. Adding a polar aromatic oil to the feedstock of a SHC reactor is effective in reducing the coke formation as described in U.S. Pat. No. 5,755,955. Furthermore, U.S. Pat. No. 6,004,453 describes such SHC processing with recycle of both heavy gas oil and unconverted pitch to enable the operation of the unit at a higher conversion, thus facilitating residue upgrading.
There is a continuing need, therefore, for improved processes and apparatuses for upgrading residue feed stocks in slurry hydrocracking and in suppression of mesophase production.